The present invention relates, in general, to electronics, and more particularly, to methods of forming semiconductor devices and structure.
In the past, the semiconductor industry utilized various structures and methods to form vertical power transistors on the same substrate with lateral low breakdown voltage transistors. Forming both a vertical power transistor and a lateral transistor on the same semiconductor die was highly desirable and offered designers the flexibility to combine various functions on a single semiconductor die. The vertical power transistor usually had a higher breakdown voltage than the lateral transistors. In order to obtain the higher breakdown voltage, the vertical transistor was formed in a thick epitaxial layer that had a high resistivity. One accepted figure of merit for the resulting on-resistance of the vertical transistor was the on-resistance area product. The vertical transistors typically had an on-resistance area product that was no less than one (1.0) milli-ohm-cm2. The lateral transistor had to be formed in a well in the epitaxial layer. The large thickness of the epitaxial layer provided sufficient depth to form the high resistivity well for the lateral transistor within the epitaxial layer.
In some applications it would have been advantageous to build medium or low breakdown voltage vertical power transistors on the same die with lateral transistors. Medium and low breakdown voltage vertical power transistors generally had a breakdown voltage no greater than about forty volts (40 V). However, medium and low breakdown voltage vertical transistors used a thinner epitaxial layer than the high breakdown voltage vertical transistors. A thinner epitaxial layer generally did not allow sufficient room to form the well in which the lateral devices were built. Consequently, it was difficult to combine any lateral transistors on the same semiconductor die with medium or low breakdown voltage vertical power devices. Accordingly, it is desirable to have a method of forming a medium or low breakdown vertical power transistor and lateral transistors on the same semiconductor die.
For simplicity and clarity of illustration, elements in the figures are not necessarily to scale, and the same reference numbers in different figures denote the same elements. Additionally, descriptions and details of well known steps and elements are omitted for simplicity of the description.